On-Site Detection of Thiamethoxam Using a Portable FRET Aptasensor with Upconversion Nanoparticles and Graphene Oxide

Thiamethoxam (THI), a widely used neonicotinoid pesticide, is of crucial significance for the accurate detection of THI in terms of environmental and food safety. In this study, a portable fluorescent aptasensor was developed for the rapid and sensitive detection of thiamethoxam (THI). Based on the principle of fluorescence resonance energy transfer (FRET), the sensor system consists of THI-specific aptamers (UCNPs-apt) and upconversion nanoparticles (UCNPs) functionalized with graphene oxide (GO). Compared to conventional fluorophores, UCNPs exhibit minimal background fluorescence, thereby minimizing interference in complex matrices such as food samples. Key parameters, including GO concentration (0.15 mg/mL), UCNPs/GO incubation time (40 min), pH (7.4), and THI reaction time (30 min), were optimized to maximize fluorescence quenching and target-induced recovery. The sensor showed a linear detection range of 0.1–100 ng/mL and a limit of detection (LOD) of 0.088 ng/mL, with excellent selectivity and reproducibility. The method was validated using peach and mulberry leaf samples, yielding results consistent with ELISA (recoveries: 98.79–113.04%; relative standard deviations: 0.46–1.92%). The integrated portable system, leveraging aptamer specificity and compact instrumentation, enables on-site monitoring and addresses the urgent need for rapid field-deployable detection of THI residues.